1. Field of the Invention
This invention relates to a resistance welding electrode chip, and more particularly to a resistance welding electrode chip having improved durability.
2. Description of the Prior Arts
The conventional spot welding electrode chips have a configuration as shown in FIG. 6, and are made of a copper based alloy like a chromium-copper alloy and a beryllium-copper alloy. A special type of the conventional electrode chips is shown in FIG. 7, and has a dispersion strengthened copper alloy insert 100 having good durability buried at the front end.
And as set forth in Japanese Unexamined Patent Publication No. 38856/1972, a resistance welding electrode chip made of a titanium carbide-copper-molybdenum sintered alloy has been known, which contains molybdenum from 0.8 to 1.5% by weight to improve the durability, and titanium carbide from 25 to 40% by volume, or equivalent to from 16 to 31% by weight, to improve the thermal shock resistance.
Another resistance welding electrode chip as disclosed in Japanese Unexamined Patent Publication No. 100986/1983 has been known. It is sintered and molded of a composite material. The composite material is composed of a carbon powder or fiber plated with copper.
Further, there has been a resistance welding electrode chip having a front end made of a composite material having wear resistance, i.e. copper or copper based alloy and a carbon fiber, as set forth in Japanese Unexamined Patent Publication No. 144386/1980.
Furthermore, as set forth in Japanese Unexamined Patent Publication Nos. 227998/1985, 231596/1985 and 231597/1985 and shown in FIG. 8, there has been a resistance welding electrode chip having a wear resistance protective surface coat 200 formed thereon.
When spot welding plain mild steel plates with the prior art electrode chips as shown in FIG. 6, the front ends of the electrode chips are softened and deformed by the high temperature, thus the area of the front ends of the electrode chips increases and the electric current density at the front ends of the electrode chips decreases. As a result, a failure welding often occurs at 5000 to 8000 times of spot welding, and the electrode chips must be re-molded. And when spot welding zinc-plated steel plates with the prior art electrode chips, the copper of electrode chips and the zinc of zinc-plated steel plates are alloyed, and the front ends of electrode chips heavily wear in a degree of 5 to 10 times that of spot welding the plain mild steel plates. Consequently, the front ends of electrode chips must be machined and ground frequently. In addition, the front ends of the electrode chips wear much more heavily when spot welding the plain mild steel plates and the zinc-plated steel plates in turn.
The resistance welding electrode chip as set forth in Japanese Unexamined Patent Publication No. 38856/1972 has a short life. As the whole electrode chip is made of the wear resistance material, the portions adjacent the front end exhibit poor electric conductivity and at which a greater amount of heat generates during the application of electric power. Thus, the strength of the portions adjacent the front end decreases sharply and deforms inevitably. And the resistance welding electrode chip as set forth in Japanese Unexamined Patent Publication No. 100986/1983 has the same problem.
Further, although the resistance welding electrode chip disclosed in Japanese Unexamined Patent Publication No. 144386/1982 has the insert 100 (FIG. 7) made of the wear resistance material and buried at the front end thereof, the insert 100 comes off or even breaks.
Furthermore, the resistance welding electrode chips of Japanese Unexamined Patent Publication Nos. 227998/1985, 231596/1985 and 231597/1985 have the surface coated with the protective surface coat 200 (FIG. 8) made of the wear resistance material, but the protective surface coat 200 comes off during the resistance welding. And it comes off also during regrinding the electrode chips to a desired front end shape.
Finally, it requires an increased number of manufacturing processes to manufacture the resistance welding chips of Japanese Unexamined Patent Publication Nos. 144386/1980, and 227998/1985, 231596/1985 and 231597/1985, respectively shown in FIGS. 7 and 8. Accordingly, they are expensive, but have problems as mentioned above.